This invention relates generally to methods and apparatus for assessing material properties and more specifically to methods and apparatus for assessing material properties of paper and other sheet-like materials.
The paper industry has for several years utilized various types of ultrasonic testing methods in order to evaluate certain material properties of paper, such as elastic properties (e.g., stiffness). Depending on the intended application, it may be desirable for the elastic properties (e.g., stiffness) of the paper to be substantially the same in all directions, in which case the elastic properties are said to be isotropic. Alternatively, the elastic properties of the paper may vary with direction, in which case the elastic properties are referred to as being anisotropic. One technique for measuring the elastic properties of paper involves measuring the speed of an elastic wave as it moves between 2 points on the material. Since the speed of an elastic wave in paper is related to the elastic properties (e.g., stiffness) of the paper, measurement of the time it takes for an elastic wave to travel between the two points on the paper can be used to ascertain the elastic properties of the paper along the line defined by the two points. An assessment of the elastic properties of the paper along several directions may be ascertained by performing the elastic wave measurement technique between several pairs of points at several different angles. For example, it is common to perform the elastic wave measurement technique in the so-called xe2x80x9cmachine directionxe2x80x9d as well as the perpendicular or xe2x80x9ccross directionxe2x80x9d in order to assess the elastic properties of the paper in these two directions.
An example of a method and apparatus for performing the above-referenced measurement technique involves the use of a contact-type piezoelectric emitter and a detector. The piezoelectric emitter may be placed in contact with the paper at a central location. The detector may then be placed at one of a plurality of discrete locations along a circle surrounding the emitter. Measurements may then be taken at each discrete location around the emitter by using the detector to calculate the time it takes a given elastic wave to travel from the emitter to the detector. The travel time may then be used to determine the velocity of the elastic wave along that particular direction. Thereafter, the velocity differences along specific directions may be used to assess the elastic properties of the paper along several different directions.
While the foregoing method for measuring the elastic properties of paper is effective from a functional standpoint and is widely used, it is not without its drawbacks. For example, such a point measurement technique is slow and difficult to automate because the detector must be positioned or moved to each of a number of discrete locations about the emitter in order to detect the velocity of the elastic wave along all directions of the paper sample being tested. Another disadvantage associated with the foregoing measurement technique is that it is not particularly well-suited for real time or xe2x80x9con the flyxe2x80x9d measurements. Consequently, to date, such point contacting methods have not been successfully implemented on a moving paper stream, such as may be found in a paper manufacturing plant. Instead, a sample of paper must be removed from the paper stream and separately analyzed. Of course, if the elastic properties of the paper in any direction are found to be non-compliant with specifications, a substantial portion of the paper from the sample lot may need to be separately analyzed to determine the extent of the non-compliance. In extreme cases, the entire lot may need to be scrapped.
Consequently, a need exists for a method and apparatus for more readily determining certain material properties of paper and other sheet-like materials that does not suffer from the shortcomings of prior art methods and apparatus. Additional advantages could be realized if such a new method and apparatus could be more readily implemented in a production environment to provide a real time assessment of material properties (e.g., stiffness), thereby making it easier to provide xe2x80x9con-the-flyxe2x80x9d measurement and/or adjustment of the tested properties or parameters.
Apparatus for producing an indication of a material property of a sheet-like material according to the present invention may comprise an excitation source for vibrating the sheet-like material to produce at least one traveling wave therein. A light source configured to produce an object wavefront and a reference wavefront directs the object wavefront toward the sheet material to produce a modulated object wavefront. A modulator operatively associated with the reference wavefront modulates the reference wavefront in synchronization with a traveling wave on the sheet-like material to produce a modulated reference wavefront. A sensing medium positioned to receive the modulated object wavefront and the modulated reference wavefront produces an image of the traveling wave in the sheet-like material, the image of the traveling wave being related to a displacement amplitude of the traveling wave over a two-dimensional area of the vibrating sheet-like material. A detector detects the image of the traveling wave in the sheet-like material.
Also disclosed is a method for assessing a material property of a sheet like material which may comprise the steps of: vibrating the sheet-like material to produce at least one traveling wave therein; directing an object wavefront toward the vibrating sheet-like material so that the object wavefront is modulated by the vibrating sheet-like material to produce a modulated object wavefront; modulating a reference wavefront in synchronization with the vibrating sheet-like material to produce a modulated reference wavefront so that a difference frequency between the modulated object wavefront and the modulated reference wavefront is within a response range of the sensing medium; combining the modulated object wavefront and the modulated reference wavefront within the sensing medium to produce an image of the traveling wave in the vibrating sheet-like material, the image of the traveling wave being related to a displacement amplitude of the traveling wave over a two-dimensional area of the vibrating sheet-like material; detecting an image of the traveling wave in the vibrating sheet material; and assessing the material property of the vibrating sheet material based on the detected image of the traveling wave.